An optical element such as a light emitting element or a photoreceptor has a structure obtained by skillfully combining regions which have different optical and electrical properties. In addition, the production necessarily takes place by an epitaxial technique since such a structure must be formed by a single layer of crystals on a base of already completed crystals. Epitaxial crystals are classified into homoepitaxial crystals and heteroepitaxial crystals, depending on whether a substrate and an epitaxial layer formed on the substrate are of the same material or of different materials.
In a conventional compound semiconductor heteroepitaxial wafer, upper, lower, left and right portions of a substrate having a diameter of two inches, for example, are cut off to shape the substrate into a rectangular configuration and thereafter an epitaxial growth is caused on the rectangular substrate by a liquid phase growth method or by a vapor phase growth method. These methods take into consideration the technical restriction on an epitaxial growth on the compound semiconductor substrate. However, such a conventional rectangular epitaxial wafer has had the following problems. One problem is the fact that the thickness of a resist film applied by spin coating or the like, is heterogenized at corner portions of the outer periphery in a pattern formation such as photolithography whereby a good pattern formation cannot be achieved in these corner portions and the yield is reduced. Another problem is the fact that the corner portions tend to crack or chip in response to a physical impact or the like due to the rectangular configuration. Hence, the manufacturing yield is reduced.
As a method of solving such problems, it is possible to use an epitaxial wafer having a circular configuration. A high yield has actually been obtained by employing a circular substrate for a silicon semiconductor. However, the present inventors have found that a high yield cannot be achieved by directly employing a compound semiconductor heteroepitaxial wafer having a circular configuration. This finding is dissimilar to what is known regarding silicon semiconductors, as will now be described.
In a compound semiconductor, it is rather difficult to grow single crystals while completely maintaining a stoichiometric composition, as compared with crystals of a simple substance such as silicon. Thus, bulk crystals of a compound semiconductor are generally inferior in their completeness to silicon. For example, in a generally used substrate of a compound semiconductor having a circular configuration, the defects are concentrated in its outer periphery, and the defects propagate toward an epitaxial crystal layer if epitaxial growth is caused on this peripheral portion. It has been recognized that the defects of the outer peripheral portion, particularly slip defects, propagate and extend toward a wafer center portion, which initially is absolutely nondefective in itself. This propagation after epitaxial growth is due to thermal stress and physical stress during a working step performed on a semiconductor device. A detailed description of slip defects is provided in J. Appl. Phys. 52(2), February 1983, p. 666-672, for example. The definition of slip defects is directed to at least ten crystal defects aligning on a straight line of 1 mm.